The present invention relates to water-based hydraulic fluids, and in particular, to those water based hydraulic fluids which contain synthetic thickeners.
Petroleum oils have traditionally been used as hydraulic fluids. Such oils exhibit Newtonian viscosity behavior. A Newtonian fluid is a fluid that possesses a viscosity which is independent of the velocity gradient. Thus, the shear stress (.tau.) is related to the shear rate (.gamma.) by the equation: EQU .tau.=.eta..gamma.
wherein .eta. is the shear rate independent viscosity. Further, petroleum oils have a viscosity that is fairly constant throughout the lifetime of the fluid at prolonged high shear rates. This mechanical stability to shear degradation is a desired property of hydraulic fluids. The shear stable Newtonian viscosity of a typical hydraulic oil is generally in the range of 10 to 100 centistokes at 100.degree. F.
Water-based lubricant products are gaining popularity due to shortages of petroleum base supplies, environmental concerns caused by problems in disposing of oil-based wastes, cost incentives and fire safety considerations. Typically, a water-based hydraulic fluid consists of several water-soluble or emulsifiable additives such as corrosion inhibitors (alkanolamines), lubricity aids (long chain carboxylic acid salts) and/or extreme pressure additives (zinc dialkyldithiophosphates, phosphate esters, borates, etc.). However, such an additive package has a viscosity that is essentially equal to that of water. It is desirable to thicken such a water-based lubricant with a thickening agent to overcome the problems associated with the use of a low viscosity fluid.
Increased viscosity of the water-based hydraulic fluids is desirable for several reasons. In particular, thickened fluid can aid in the operation of system valves which have been designed to work specifically with oil-based fluids. Further, thickened fluids are less prone to experience leaking through small holes or cracks in the hydraulic system. Higher pump efficiencies are obtainable with thickened fluids, especially at high loads, and such fluids exhibit wear prevention characteristics in both hydrodynamic and elastohydrodynamic wear modes. Thus, water-based hydraulic fluids are typically prepared using viscosifying amounts of polymeric thickeners.
Unfortunately, hydraulic fluids are subjected to high rates of shear, often in excess of 10.sup.6 sec.sup.-1. Such high rates of shear can rapidly mechanically degrade efficient, high molecular weight polymeric thickeners. This irreversible shear thinning yields hydraulic fluid formulations containing polymeric materials having lower molecular weights which are less efficient thickeners. Thus, the viscosity of such a formulation containing a polymeric thickener will decrease after periods of use. Viscosity loss due to shear degradation can be minimized by employing low molecular weight polymeric thickeners. However, such low molecular weight polymeric thickeners are not efficient thickeners and require large amounts of polymer in order to obtain a formulation exhibiting the desired viscosity.
In view of the deficiencies of the prior art, it would be highly desirable to provide a water-based hydraulic fluid composition which is highly shear stable, does not experience substantial viscosity loss upon increases in temperature and is capable of providing acceptably low amounts of wear in pumping apparatus during use.